Conventionally, as an impeller used in a rotary machine such as a compressor, a welded impeller is known.
For example, as shown in FIGS. 10A and 10B, a welded impeller 10 formed in one body by mutually welding constituent members such as a cover 1, blades 2, and a disc 3 (showing weld zones with a symbol 4) is known.
Thus, such a welded impeller 10 is manufactured, as shown in FIG. 11, by independently preparing the cover (shroud) 1, the blades 2, and the disc (hub) 3 (S1), integrally forming these constituent members by weld zones 4 (S2), performing heat treatment such as quenching and tempering for the purpose of removing residual stress after the welding and enhancing hardness and durability (S3), performing finishing using mechanical working and electrical discharge machining (S4), and then performing trimming and polishing using electrolytic polishing and chemical polishing (S5).
In recent years, the impeller has made progress toward high efficiency and high performance, and a shape thereof has necessarily become complicated, diversified, and increased in precision. In the case of the welded impeller, the heat treatment such as quenching and tempering for the purpose of removing residual stress after the welding and enhancing hardness and durability is performed (S3), but during the welding (S2) and the heat treatment (S3), deformation occurs leading to the possibility of dimensional precision being reduced.
For this reason, for the purpose of reducing the deformation caused by the welding and the heat treatment and improving the dimensional precision, for example, as disclosed in Japanese Unexamined Patent Application, First Publication Nos. 2004-92650, 2009-156122, and 2010-230012, an integrated impeller (denoted by a symbol 11) integrally forming the cover, the blades, and the disc without being welded is provided.
In the integrated impeller 11 disclosed in these Patent Literatures, as shown in FIGS. 12A and 12B, a cover (shroud) 5, blades 6, and a disc (hub) 7, which are main components, are manufactured from a metal serving as a base metal in a predetermined shape by mechanical working and electrical discharge machining, and are free from deformation caused by the welding. For this reason, a precise shape for obtaining desired performance as the impeller can be obtained, and no defects associated with the welding occur.
Thus, the integrated impeller 11 is manufactured, as shown in FIG. 13, by preparing the cover (shroud) 5, the blades 6, and the disc (hub) 7 from a material made of one block by means of cutting (S6), performing heat treatment for the purpose of hardening a surface and improving toughness (S7), performing finishing using mechanical working and electrical discharge machining (S8), and then performing trimming and polishing using electrolytic polishing and chemical polishing (S9).